1. Field of the Invention
This invention relates in general to power-line communication systems and, more specifically, to such communication systems employing baseband digital communication techniques.
2. Description of the Prior Art
Today's electric power systems for generating and transmitting electricity consist primarily of generating units, transmission lines, and major switching substations all interconnected in a giant electrical grid extending for hundreds of miles. Power is taken from the grid at the major substations where it is fed to an intricate distribution network. Finally, branches of the distribution network provide the electric power to the customer's premises. Centralized monitoring and controlling of the entire generation and transmission system is essential to satisfy the many operational goals of the system. Included among these objectives are: analysis and prediction of load flow among system elements, economical control of power flow and generation, network switching to minimize the impact of unexpected outages, and prediction of the effects of various switching operations on network stability and security. Achievement of these objectives necessitates the use of a large central computer constantly receiving data from diverse locations on the power grid and responding with appropriate control signals.
The system for performing centralized control of a power network is often referred to as the supervisory control and data acquisition (SCADA) system or the energy management system (EMS). Typically, the elements of an EMS include: a central computer, data and voice communication channels connecting the computer with each of the generating plants and major switching stations, and a remote terminal unit (RTU) at each generating and switching station. The remote terminal unit monitors apparatus as its location, digitizes analog measurements, transmits data to the computer, and receives and executes computer commands.
Three communication schemes are in wide use today for connecting the EMS central computer with the RTU's. Utilities use leased or dedicated telephone lines, private multiplexed microwave system, or power-line carrier communication systems. Each of these communication techniques is discussed in detail and compared to the novel power-line baseband communication system hereinbelow.
Many electric utilities use dedicated telephone circuits, leased at considerable expense from the local telephone company, as communication channels for EMS purposes. The EMS digital messages are converted to or from modulated audio-frequency carrier signals by modems at each end of the telephone line. These modems typically communicate at 1200 to 4800 bits per second on each line. The utility user provides special interface facilities for the mutual protection of the utility's modems and the telephone plant; the lessor is responsible for other aspects of service quality and circuit maintenance.
Today, an increasing number of energy management systems employ multi-link microwave systems which are planned, purchased, installed, and maintained by the utility user. Although quite costly, a microwave system provides ample capacity for present and future utility needs. Discrete audio-frequency channels, similar in characteristics to telephone circuits, are multiplexed by tens or hundreds into a single microwave system. Voice signals may be transmitted directly; data are transmitted via modems similar to those used with leased telephone lines. The user has complete control over the system, but also assumes responsibility for maintenance, notably including repeater installation. The repeaters are frequently required at remote locations having difficult access not otherwise needed for utility operations.
The third alternative, a power-line carrier communication system, involves the transmission of longwave carrier-modulated signals directly over the power transmission line. System transmitters generally operate at a carrier frequency in the range of 30 kHz to 300 kHz. Frequencies below 30 kHz are unusable due to the difficulty of building associated equipment to operate below this limit. There is also a substantial increase in received noise power below 30 kHz. Frequencies greater than 300 kHz suffer substantial signal attenuation on the line and increased radiation of the carrier signal, raising the possibility of interference with long-wave radio services. For operation in the 30-300 kHz range the radio frequency output power of these carrier transmitters is typically 1 watt or 10 watts. For critical applications in high-noise environments 100-watt transmitters have been used. Simple modulation schemes are generally employed, i.e., on-off keying or frequency-shift keying. Each modulated carrier signal typically occupies approximately 3 kHz of the frequency spectrum thereby permitting, in theory, multiplexing of approximately 90 individual signals in the 30 kHz to 300 kHz band. Practical problems of adjacent channel interference usually limit this number to much less than 90.
The power-line carrier communication receiver must have the following characteristics for proper detection of the carrier: adequate selectivity, long-term reliability, and reasonable security against inadvertent operation or reception errors due to line noise or switching transients. Of course, the power level of the received signal must be substantially greater than the noise power on the transmission line to allow proper reception and demodulation.
Other components of the power-line carrier communication system include: a coupling capacitor to couple the carrier signal to the energized transmission line while protecting carrier equipment and personnel from large 60 Hz voltages on the line; line traps inserted on the transmission line to prevent any portion of the transmitted signal from propagating to other transmission lines via a power bus to which several transmission lines are connected, and to prevent faults on the power bus or other transmission lines from shorting the carrier signal; and matching networks to match, in conjunction with the coupling capacitor, the nominal transmitter and receiver impedance to the power line characteristic impedance. In a typical power line carrier communication system installation a transmitter, a receiver, and their associated components are connected at each terminal of the transmission line.
In recent years, use of the power-line carrier technique as the communication link in the EMS has diminished. Although it provides a highly reliable, utility-controlled medium for EMS data transmission, it is not often utilized in new installations because of the lack of adequate carrier spectrum for providing needed data transmission capacity to a large number of substations and generating plants. The increasing use of carrier for protective-relaying signals accentuates this shortage. Before discussing the application of power-line carrier communication to protective relaying, it is necessary to understand protective relaying fundamentals.
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits which could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a faulted condition, line currents can increase to several times the normal value thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in the range of 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers interrupting up to 80,000 A and the completion of backup operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which monitor ac voltages and currents, to locate line faults and initiate via tripping of appropriate circuit breakers. These faults are located by detection of abnormal relationships of ac voltages and currents.
Protective relay systems for transmission lines consist of measurement apparatus at each transmission-line terminal or substation and bidirectional communication links connecting the relays. The devices at each transmission-line terminal compare fault location information to quickly determine if the line fault is on that segment of the transmission line between them. If the comparison indicates that the detected fault is internal, i.e., between the two protective relays, the intervening transmission-line section is isolated by tripping associated circuit breakers. If the comparisons indicate that the fault is not between the two protective relays, the circuit breakers remain closed. This protection technique is known in the art as pilot relaying.
The power-line carrier communication system, previously discussed for use on an EMS, initially evolved for use as a bidirectional communication channel of a protective-relaying network. A power-line carrier receiver and transmitter, and their associated equipment, are connected to each protective relay to serve as an incoming and outgoing communication link. Since the percentage of time during which faults are actually present on the line is small, the transmitters and receivers are often equipped with auxiliary voice modulation and demodulation equipment to provide voice channel communication between the substations where the protective relays are located. The system will interrupt voice channel communications if a power-line fault occurs during a conversation. A power-line carrier communication system for protective relays is disclosed in U.S. Pat. No. 4,205,360.